Oyster Restoration in Virginia

Oyster restoration began on a large scale in Virginia at the start of the 20th Century, by private individuals who had an economic interest in growing oysters for profit.

In 1892-94, the Baylor Surveys in Virginia delineated areas where oysters were growing naturally and identified submerged lands that were barren of oysters. (Maryland defined its Natural Oyster Bars in surveys by C.C. Yates between 1906-1912.) In Virginia, these barren areas were made available for lease (privatized) at very low cost per acre.

Private aquaculture used the same techniques as the state used on public oyster grounds. However, the right to harvest the oysters that grew on "private rocks" (submerged land leased by the Commonwealth of Virginia) was limited to the leaseholder.

Leaseholders bought old oyster shells from packing houses or dredged them from fossil oyster reefs in the James River. The shells were shipped to privately-leased submerged tracts, then stocked with young seed oysters that were often harvested from the living reefs on the James River.

After a year or two, the seed oysters grew to market size and were harvested by the leaseholder - assuming an unauthorized oyster pirate had not scraped up those oysters illegally. The Ballard Fish & Oyster Company on the Eastern Shore still advertises that is has been "producing great shellfish since 1895."1

The state used public funding to restore natural, still-producing oyster bars within the areas delineated in the Baylor Surveys. Oyster shells were dumped on the natural oyster bars, not the barren submerged lands identified in the Baylor Surveys and then leased for private aquaculture initiatives. The "public rocks" were available for public harvest by watermen who could not afford to pay for private leases and develop "private rocks."

the Baylor Surveys defined the boundaries of public oyster grounds in the Southern Branch of the Elizabeth River
Source: National Oceanic and Atmospheric Administration (NOAA), Norfolk: Public Oyster Grounds (1892)

Expanding the square footage of hard substrate created more suitable oyster habitat on the river bottom. More oysters grew, but the state did not impose tight restrictions on harvest. Tonguing and dredging dispersed the shells on the public rocks, reducing three-dimensional reefs into scattered piles. That limited natural oyster reproduction. Restoration on public rocks became a put-and-take fishery, comparable to annual stocking of trout in mountain streams for recreational fishing.

The state investment in oyster restoration on public rocks was justified in part because it created economic opportunities for the watermen and the influential oyster shucking houses. The public paid for increasing the oyster supply, though only few private individuals quickly harvested the benefits. The oyster restoration process was a subsidy for a traditional industry. The put-and-take restoration approach was not a cost-effective was to increase the oyster population,, but it was politically popular in the thinly-populated counties along the Chesapeake Bay.

Because the new oysters were quickly harvested, restoration efforts failed to increase the overall population of oysters in the Chesapeake Bay. Few people considered re-establishing oyster bars without harvesting the oysters, just to provide filter-feeders that could help to re-balance the ecology of the Chesapeake Bay.

The political and economic reasons for authorizing harvest of restored sites was joined by a biological reason, starting in the 1950's. The single-celled protozoan parasite known as Dermo (Perkinsus marinus) was first identified in the Chesapeake Bay in 1949. A decade later, the parasite MSX (Haplosporidium nelsoni) infected oysters where salinity was at least 15 parts per thousand (15ppt).

Both diseases killed oysters before they reached marketable size, but oyster beds in the Rappahannock River survived better. The spring runoff of freshwater in that river reduced salinity below the thresholds acceptable to the Dermo and MSX parasite in April/May, at a time when water temperature exceeded 15°C. In other streams, the timing of the salinity drop occurred when the water temperature was lower and the parasites were still dormant, or after the water temperature had climbed to the point where diseases had already damaged the oyster population.2

Since most oysters were dying within the first three years of their life, the restocked beds would be "wasted" if not harvested quickly. However, any oysters with potential to survive sediment, predation by cow-nosed rays, and disease were removed by quick harvest. As a result, restocked oyster beds never had a chance to regrow to their previous extent (and the opportunity for oysters to evolve resistance to the diseases was lost with each harvest).

Commercial harvest dropped from 4 million-7 million bushels/year in 1880-1925 to a low point of just 18,000 bushels in 1995-96. That triggered a change in oyster restoration policy.3

To increase the oyster population, Virginia and its partners in the Chesapeake Bay Program adopted the new strategy of creating oyster sanctuaries, where harvest was 100% prohibited. There are ecological reasons for oyster restoration separate from the economic objectives to increase the commercial harvest.

More filter-feeding oysters (each one processing 3-12.5 gallons of water daily) could improve the water quality in the Chesapeake Bay. Oysters had the potential of stripping some excess nutrients from the water column. The oyster restoration project manager for the Chesapeake Bay Foundation commented:4

They are their own living water treatment plants... Healthy oyster beds are good for a lot more things than just eating oysters.

Adding more oysters into the waters of the Chesapeake Bay could help Virginia come closer to meeting the Clean Water Act requirements. If the oysters were protected by sanctuaries, they could help "Save the Bay." If the oysters were harvested and enjoyed by consumers who do not live close to the water, the oysters might stimulate public support for public policies and investments that have even greater impact on pollution that filtering the water.

The Chesapeake Bay Agreement signed in 2000 included a specific goal for using protected areas as a technique for restoring oysters:5

By 2010, achieve, at a minimum, a tenfold increase in native oysters in the Chesapeake Bay, based upon a 1994 baseline. By 2002, develop and implement a strategy to achieve this increase by using sanctuaries sufficient in size and distribution, aquaculture, continued disease research and disease resistant management strategies, and other management approaches.

Virginia Beach has acquired its own oyster leases on the Lynnhaven River and Broad Bay to support restoration projects.

Since the 1800's, the standard process for growing oysters in the bay has been to scatter oyster shells on the bottom and wait to see if the site produced mature oysters in two-three years. In 1993, the Virginia Marine Resources Commission (in cooperation with the Virginia Institute of Marine Science) began to build three-dimensional vertical reefs, rather than just spreading a thin layer of shells on the bottom.

The reefs were created in new sanctuaries established at the mouth of the Piankatank River. Since 1963 the site had been managed for producing seed oysters, which were harvested soon after larvae "set" and then moved to other locations where the oysters grew into maturity.

granite rock, brought in by barge, was methodically placed in the Piankatank River near Gwynn's Island in Mathews County
Source: US Army Corps of Engineers, Norfolk District Image Gallery

It was a good location for generating seed oysters because, within the embayment at the river mouth, brackish water flowed in a gyre. That pattern trapped oyster larvae and created high potential for a productive reef. In addition, water quality was good, since there was so little development upstream in the Piankatank River watershed. The state created Palace Bar Reef and designated it as a sanctuary, banning harvest to ensure that dredging/tonguing would not alter the vertical structure by scraping off the top layer.6

moving granite from barge to the bottom of the Piankatank River, to provide oysters a base for building a new reef
Source: US Army Corps of Engineers, Norfolk District Image Gallery

Additional reefs have been created in that bay, and elsewhere. In the 20 years after the Palace Bar Reef project, Virginia constructed more than 100 sanctuary reefs, investing in three-dimensional structures as well as spreading shell onto flat oyster beds.

In 2012, the Chesapeake Bay Foundation expanded Palace Bar Reef again, but used a different technique. It added hollow concrete spheres ("reef balls") already covered with spat, after sinking the balls in the Virginia Institute of Marine Science oyster hatchery tanks. Reef balls were used to create a three-dimensional substrate rather than natural shell, because the supply of oyster shell could not meet the demand for restoration. In 2014, The Nature Conservancy committed funding to create a new reef nearby in partnership with the U.S. Army Corps of Engineers and the Virginia Marine Resources Commission, this time using recycled concrete.7

Palace Bar Reef, at the mouth of the Piankatank River, was the first three-dimensional reef for oyster restoration in the Chesapeake Bay
Map Source: ESRI, ArcGIS Online

In the Rappahannock River, the state built 14 one-acre sanctuary reefs. Broodstock (mature oysters capable of spawning) was planted and then protected from disturbance. In additional, 500 acres of two-dimensional bars were constructed, scattering shells on the Rappahannock River bottom that would provide habitat for the larvae generated naturally on the different sanctuary reefs.

The presence of new reefs and bars with large oysters, grown to marketable size, led to political pressure to permit commercial harvest at those sanctuaries. The success of the restoration forced Virginia to find a middle ground between "let the watermen harvest everywhere" and "keep the oysters undisturbed forever."

Virginia implemented an oyster management solution that was intended to balance the ecological objectives of increasing oyster populations with economic opportunity. The 500-acre harvest area on the Rappahannock River was divided into six sections.

Two sections were opened for harvest every three years, allowing oysters to grow for two years undisturbed before a section was harvested on the third year. In addition, watermen were required to place the largest oysters on the sanctuary beds as broodstock, rather than selling them to customers. The rotating schedule enhanced spawning from the three-dimensional sanctuary reefs, but still provided annual opportunity for watermen to make a profit.8

Permanently-protected oyster sanctuaries now are providing broodstock for re-stocking public oyster beds, and other public oyster grounds are being harvested on a rotating basis. The schedule allows time for young oysters to grow, then be collected before the diseases of Dermo and MSX kill the mature oysters. The management scheme involves having enough beds in the harvest phase each year to keep watermen busy collecting large oysters, reducing the temptation to poach small oysters from closed areas. As described in a 2011 news release from the governor, celebrating the rebirth of the industry in Virginia:9

Harvest areas are opened on a staggered basis for one harvest season then closed for one or two years in order to give oysters a chance in those areas to grow to market size. Individual harvest area openings are staggered on a two or three year rotational basis. This allows harvests in some areas while others remain closed so the oyster stocks can regenerate and be reopened later, in time to harvest the market-sized stocks before the diseases kill them.

Sanctuary reefs create masses of interconnected oysters, which are more capable of resisting predation by cow-nosed rays than the two-dimensional bars. In 2006, after scientists planted 750,000 oysters on a flat bar in the Piankatank River and even added a layer of shells to protect the new oysters, a school of cow-nosed rays ate 94% of them in just five days.

One option considered by state officials was to create a market for eating cownose rays. The objective was to entice fishermen to catch the rays, so fewer rays would eat fewer oysters. It was an approach comparable to efforts on land to increase the number of preferred game animals such as deer/elk, by reducing the number of predators such as wolves/coyotes. Another major predator of oysters, the blue crab, has been managed through commercial harvesting for many decades.

The cownose rays were advertised as Chesapeake Rays, leading to one headline "Help the Bay, eat a Chesapeake ray." However, no population assessment was completed to determine if a cownose ray fishery would be sustainable, and a decade later new research revealed that they were not major predators of oysters:10

In fact, cownose rays prefer clams over oysters to begin with, and softer shelled mollusks in general, as large oysters are too big and tough for the rays to eat.

A five-year experiment at building new reefs in oyster sanctuaries on the Great Wicomico River showed that tall (10-18 inch) reefs were more successful in recruiting new oysters than short (3-5 inch) reefs. Tall reefs benefited from currents removing sediments and providing more algae for oysters to eat, and prohibiting harvest preserved the vertical structure.

In harvested areas, the natural reef is lowered by tonguing and dredging as oysters are scraped away. The areas open to harvest typically have 2-11 oysters/square meter, while the short reefs built in the Great Wicomico River sanctuary developed 250 oysters/m2 and the tall reefs had 1,000 oysters/m2. The Corps of Engineers defines restoration as "successful" if the live adult oyster density is at least 50/m2.

The restoration effort on the Great Wicomico River produced dramatic results, in part because water circulation patterns in that area tend to trap oyster larvae which then could set on the artificial reefs:11

The Great Wicomico's re-established population, which the researchers estimate at 184.5 million oysters, is the largest of any native oyster population worldwide. The restored population, which exists on 86.5 acres of reefs, is roughly equivalent to the entire oyster population on all of Maryland's 270,000 acres of public oyster grounds.

less than 100 acres in an oyster sanctuary on Great Wicomico River has more oysters than all 270,000 acres of public oyster grounds in Maryland, demonstrating the productivity of vertical reefs undamaged by harvesting
Source: ESRI, ArcGIS Online

Oyster sanctuaries and public oyster grounds are being replenished with shells dredged from various sites, including the Piankatank River. The primary source for shell has been old reefs on the James River that were once highly productive. As sea level has risen since the end of the last Ice Age, those fossil reefs are now located too deep underwater to sustain oysters, so the shells are being redistributed to other sites across the bay watershed.

However, the demand for shells exceeds the supply, so other material has been used to create artificial reefs. Ground concrete was scattered on the bottom of the Piankatank River to build the foundation for a new oyster reef there.

the Piankatank River oyster reef was started with ground-up concrete
Source: US Army Corps of Engineers, 140530-A-OI229-047

a Lafayette River oyster reef was built with fossilized oyster shell
Source: US Army Corps of Engineers, 140314-A-ON889-108

Restoration of public sites, and creation of new habitat on private leases, has helped increase the population of oysters in the bay. Watermen are harvesting increasing numbers of bushels from both "public rocks" in the winter, and from private leases on submerged river/bay bottom in the summer.

In 2012-13, the Virginia harvest increased to 400,000 bushels, then 500,000 bushels in 2013-14.

Private aquaculture based raising oysters in cages, a practice that has grown substantially since 2008, is playing a key role in re-establishing the oyster industry in Virginia. By 2018, Virginia had leased over 130,000 acres of bottom for raising oysters.12

Modern oyster gardening does not require the old techniques of harvesting with tongs and dredges on the open water during the October-February season (extended into April on the James River). Restoring the oyster through aquaculture benefits the ecology of the bay and economics of waterfront towns, but growing oysters in cages rather than on oyster beds will not preserve the traditional skills or tools of Virginia's remaining 1,000 watermen.13

Virginia, Maryland, and Federal officials have a common perspective on the science of oyster restoration, but differ on the politics and priorities.

For a century, Maryland focused on managing oyster beds that were open to public harvest, while Virginia enabled private leases for creation of new oyster beds on submerged land that was no longer naturally productive. Federal officials got involved in oyster recovery as part of efforts to reduce pollution and "Save the Bay" so it met the standards of the Clean Water Act.

Initial Federal efforts in Virginia mimicked the state's approach, with the addition of new shells to public oyster beds in the Rappahannock River in 2000 and in Tangier Sound in 2002-2003. However, the Federal efforts were intended to create ecological benefits rather than support traditional harvesting by watermen.

The Corps of Engineers concluded that the state's approach (known as "repletion") was not cost-effective, because harvesting removed the oysters and damaged the oyster beds as fast as they were restocked. The Corps estimated that 1/3 of the oysters placed in Maryland sanctuaries between 2008 and 2010 were harvested illegally by poachers. Federal agencies (in addition to the Corps, the National Oceanic & Atmospheric Administration funded oyster restoration projects) stopped trying to grow oysters at public expense and plant them for private harvest, which socialized the costs while privatizing the benefits:14

Repletion appears to be a short-term, unsustainable effort which does not provide for long-term ecological recovery of the oyster and Chesapeake Bay. There is no evidence that oyster harvest areas have ever been sustainable because oyster populations cannot repair the damage.

At the end of 2018, the Virginia Marine Resources Commission made it harder for Maryland to maintained its traditional repletion program. It stopped issuing permits to transfer seed oysters to Maryland. The state agency also ended the Fall 2018 harvest of seed oysters from the James River earlier than planned, to avoid exceeding the quota of 40,000 oyster seed bushels.

Since 2011, the Virginia Marine Resources Commission had authorized a Spring harvest of 80,000 bushels of seed oysters and a Fall harvest of 40,000 bushels. At least half of the James River seed oysters were sold to be placed on the bottom of rivers and the Chesapeake Bay in Maryland.

Maryland had no source of seed oysters for transplanting. Virginia officials assumed they had a surplus on the Piankatank and Great Wicomico rivers as well as on the James River.

Virginia scientists were concerned that too many seed oysters would be harvested. Virginia watermen were concerned because the transplanted oysters reduced the supply in Virginia and increased the supply from competitors in Maryland.15

The conflict between state-Federal restoration priorities was revealed clearly in 2011, over repletion of reefs on the Great Wicomico River. The Corps and the state had cooperated on creating the state's largest artificial reef project since 2004. The sanctuary reefs were off-limits to commercial harvest, and in 2011 Virginia decided that it would not contribute its 25% share unless some acreage could be opened to watermen. Federal rules allowed restoration funding for just sanctuaries.

The ultimate compromise led to the Federal government spending only $600,000 of the planned $2.5 million for the Great Wicomico River that year. The sanctuary reefs in the Great Wicomico River were raised to a higher level by the addition of new shell, providing more habitat for oysters.16

the initial sanctuary reefs created in 2004 on the Great Wicomico River were raised by the addition of more shell
Source: US Army Corps of Engineers, Norfolk District Image Gallery

The state also rejected $200,000 for construction of new oyster reefs in the Lafayette River. The river has been closed to shellfish harvesting since 1934 due to bacterial pollution from Norfolk's sewage and stormwater runoff, so increasing the oyster population in the Lafayette River would provide no economic opportunity for commercial harvest.17

there is now an oyster sanctuary reef along the Elizabeth River shoreline next to the Corps of Engineers office building
Source: US Army Corps of Engineers, Norfolk District Image Gallery

The Lafayette River was the first target for restoration, as part of the Elizabeth River Project. The river suffered summertime red tides (algae blooms) triggered by excessive nutrients from nearby residences, but had high potential for recovery because there was little industrial development along the shoreline except for the Norfolk Naval Base at the river's mouth. In 2010, the Chesapeake Bay Foundation placed the first reef balls used for oyster restoration in Virginia into the Lafayette River, after the reef balls were stocked with spat produced at the Virginia Oyster Restoration Center in Gloucester County next to VIMS.18

Though the state was unwilling to contribute oyster shells in 2011, restoration efforts have continued on the Lafayette River. In 2014, scientists discovered evidence that vertical oyster reefs can rebuild naturally if not disturbed or harvested. An underwater survey of the Lafayette River revealed 10 natural oyster reefs populated by native oysters. Watermen had not harvested oysters from the polluted, urban river in 80 years, and somehow during that time the oyster population recovered naturally.19

In the 2014 Chesapeake Bay Agreement, Maryland and Virginia each committed to restoring oyster reefs in five rivers by 2025. The five chosen by Virginia are the Lafayette, Lynnhaven, Piankatank, Lower York, and Great Wicomico rivers. The first declaration of success was for the Lafayette River in 2018. "Success" was defined by establishing 80 acres of oyster reefs, with an oyster density of 50 oysters per square meter.

Virginia committed in 2014 to complete oyster restoration in five rivers, and achieved success in the Lafayette River in 2018
Source: National Oceanic and Atmospheric Administration (NOAA), 2018 Virginia Oyster Restoration Update

Restoration efforts created 12 new reefs totaling 32 acres with recycled shells, granite and crushed concrete on the sandy bottom. Because scientists found 48 more acres of existing reefs, the restoration effort was completed far quicker than expected. The Elizabeth River Project focused next on the Eastern Branch of the Elizabeth River.

The Lynnhaven River Now project focused on the Lynnhaven River. In 2018, there were 80 acres of oysters in the Lynnhaven River, roughly half of the goal of 150 acres.

Since government funding is directed at the five rivers targeted for restoration by 2025, private funds will be required in addition to Federal, state, and local resources. The National Fish and Wildlife Foundation gave $200,000 to Lynnhaven River Now and the Elizabeth River Project in 2018, helping those local organizations continues their recovery projects.20

The other large Federal-state debate was over Virginia's proposal to import an Asian species (Crassostrea ariakensis) to replace the native species, (Crassostrea virginica). State officials assumed Dermo and MSX had permanently damaged the Eastern oyster that had evolved in the Chesapeake Bay, while the Suminoe oyster originally from the China Sea was resistant. If the limiting factor in the Chesapeake Bay was not the loss of habitat but the pressure of disease, then introducing a disease-resistant oyster was thought to have both economic and environmental benefits.

Federal officials required completion of an Environmental Impact Statement (EIS) before authorizing the release of the Asian oyster, to assess if new virus or other diseases might be introduced at the same time. The EIS assumed the goal was to be able to harvest 5 million bushels/year from the entire bay, roughly comparable to harvests between 1920's-1970's before Dermo and MSX devastated the population. At the time of the EIS, the existing population in the Chesapeake Bay was less than 10% of the desired number (12 billion market-size oysters).21

The slow, five-year EIS process frustrated state officials, but led to the key discovery that has enabled recovery of the oyster population.

The environmental analysis required evaluating comparable versions of the Asian and native species. To protect against an accidental introduction, a sterile version of the Asian oyster was used to see how it would grow in the Chesapeake Bay. Since the Asian species had been genetically modified to have a third set of genes to make it sterile, a "triploid" version of the native species (with 30 rather than 20 chromosomes) was needed. Standish Allen, at the VIMS Aquaculture Genetics and Breeding Technology Center and the person who discovered in 1979 how to create triploid oysters, produced sterile versions of both species and distributed them for testing around the bay.

Growers discovered that the triploid version of the Eastern oyster grew substantially faster than the normal diploid version, since no energy was diverted to spawning (one reason that eating oysters was not recommended in the summer months that do not have an "R" in their name). The triploid version matured within 18 months, so fast that it was ready for harvest before being damaged by Dermo and MSX. The testing process trained a critical mass of people in how to grow oysters with intensive management using cages that floated from docks, a revolutionary change from the traditional scatter-shell-and-see-what-grows-on-the-bottom process.

After realizing that the native species was as suitable as the Asian one, the Corps - with the support of Maryland and Virginia - rejected the proposal to import the Asian species. Oyster growers, already experienced in using the gear for the Crassostrea ariakensis tests, switched to growing the triploid version of the native species. Aquaculture has boomed, and now offers one path for getting to the target of 12 billion market-size oysters in the bay.22

The Federal and state agencies proceeded to develop a master plan for oyster restoration. It made clear that "Save the Bay" efforts will not restore all 232,016 acres of the original habitat for oysters, as documented in the 1892-1894 Baylor surveys.

The plan did establish a numeric target for the area to be set aside as sanctuaries: 20-40 percent of historic habitat, equivalent to 8-16 percent of the land designated as Baylor Grounds in Virginia and Natural Oyster Bars by the Yates surveys in Maryland. The Corps clarified that it had no authority to create new sanctuaries; that was a state role. Federal efforts in Virginia would be focused on existing sanctuaries in 10 tributaries with high-salinity water, designated Tier 1 sites:23

By 2012, 2,214 acres of oyster habitat had been restored in Virginia. That was less than 2% of the 121,000 acres of restorable habitat identified by the "Native Oyster Restoration Master Plan" that was within Baylor Survey areas reserved for public use. Though oysters can build their own habitat by creating reefs, in the Chesapeake Bay the substrate is disappearing naturally (dissolving in the water, or sinking into the soft sediments) faster than oysters are replacing it.

As sea level rises, the challenge of oysters to maintain their existing habitat is increased. Human placement of granite or other hard material on submerged lands can mitigate the loss of calcium-based substrate.

In 2007, two scientists published a provocatively-titled study, "Why Oyster Restoration Goals In The Chesapeake Bay Are Not And Probably Cannot Be Achieved." They noted that it would be impossible to restore the ability of the oysters a century ago to filter the entire volume of the Chesapeake Bay in three days, and the most optimistic expectations should be a modern equivalent of 14-28 days. They identified only one location in the entire bay that was healthy enough to support regular commercial harvest, the Burwell Bay region of the James River.24

Another study in 2015 identified a previously-unrecognized threat that might limit restoration in shallow waters. The Smithsonian Environmental Research Center determined that "dead zones" can appear in creeks and rivers as shallow as 20' deep, as well as deep waters on the Chesapeake Bay. After the sun sets in the evening, oxygen is continuously removed from the shallow water by the respiration of animals and plants and decay of material on the bottom - but no oxygen is provided by photosynthesis.

When wind and currents do not stir the shallow waters sufficiently, brief periods of time without oxygen occur in shallow waters once thought to be safe places for creating new reefs. Research was conducted in the "Room of D.O.O.M." (Dissolved Oxygen Oyster Mortality), and as one restoration leader noted:25

How would you like it if we sucked the oxygen out of your bedroom for 15 minutes at night?... The shallow-water habitats, while they're not without oxygen for a long period of time, are out of oxygen long enough to do damage.

Restoration will be expensive. Costs for building 12-inch high reefs and seeding them with spat are estimated a $1.0 billion-$3.6 billion for just Virginia. Costs would be higher if sanctuaries were located in low salinity tributaries, where oyster reproduction is lower. Meeting all oyster goals for ecosystem restoration in the Chesapeake Bay Protection and Restoration Executive Order (E.O. 13508) could cost $1.6 billion-$5.4 billion in Virginia.26

Of the three causes for the decline of the oyster in the Chesapeake Bay - over-harvesting, disease, and habitat loss - restoration of habitat will be the most expensive technique to re-establish a sufficient number of oysters to meet ecological objectives. If funding for restoration is used for dual purposes, including perpetuating a "put and take" fishery where oysters are removed by commercial harvesting, then the costs will be even higher.

Aquacultural operations that raise fish create artificially-high concentrations of animals that allow disease to spread. Oyster aquaculture may have the opposite effect.

Oysters naturally establish reefs and live in high concentrations. The concentration levels are less significant than the age of the oysters for the risk of infection by Demo and MSX. Those diseases take several years to infect maturing oysters.

Oyster aquacultural operations harvest oysters before they become diseased. A study in 2018 concluded that the harvesting cycle might reduce the presence of those diseases in the wild, and:27

...oyster aquaculture could serve as a "sink," rather than a "source," for pathogens that primarily spread when oysters die, because the point of aquaculture is to remove oysters from the system before that happens,/dd>